In the field of portable digital displays it is desirable that the electronic devices to which the displays are connected be portable, low in power, capable of being viewed in dark locations or locations of low ambient light, and self contained. Liquid crystal devices have been commonly used for displays because of their low power requirement. However, an external source of light must be used for viewing a liquid crystal display. In a well-lighted room a liquid crystal display can be easily viewed using room light. In order for a liquid crystal display, for example a computer monitor, to also be viewable in a dark ambient, internal back lights have been provided as part of the monitor unit. These internal back lights can be plugged in and/or turned on when needed. The Series 6420 display provided by Alphasil, Inc., of 1045 Mission Court, Fremont, Calif., 94539-9971 includes an internal back light, for example. Electroluminescent panels used in existing displays are thin, on the order of 1.2 mm thick, available in many sizes and shapes and provide uniform light emission over their surfaces. The color and intensity of the emitted light can be changed by adding a filter.
The disadvantage of providing an internal back light as part of the monitor unit is that the size, weight, and expense of the internal back light are present in the monitor unit whether they are needed or not. Further, the power supply for the monitor must include sufficient capacity to power the back light. In some applications it is desirable to have a portable computer or hand held calculator or other device which is as light weight and small in size as possible. Since it may be only occasionally necessary to use a back light with the device, the weight, size, and power requirements of a built-in back light may be undesirable.